Universe Today

In the March 25th 2005 issue of Science Magazine, the High Energy Stereoscopic System (H.E.S.S.) team of international astrophysicists, including UK astronomers from the University of Durham, report results of a first sensitive survey of the central part of our galaxy in very high energy (VHE) gamma-rays. Included among the new objects discovered are two ‘dark accelerators’ – mysterious objects that are emitting energetic particles, yet apparently have no optical or x-ray counterpart.

This survey reveals a total of eight new sources of VHE gamma-rays in the disc of our Galaxy, essentially doubling the number known at these energies. The results have pushed astronomy into a previously unknown domain, extending our knowledge of the Milky Way in a novel wavelength regime thereby opening a new window on our galaxy.

Gamma-rays are produced in extreme cosmic particle accelerators such as supernova explosions and provide a unique view of the high energy processes at work in the Milky Way. VHE gamma-ray astronomy is still a young field and H.E.S.S. is conducting the first sensitive survey at this energy range, finding previously unknown sources.

Particularly stunning is that two of these new sources discovered by H.E.S.S. have no obvious counterparts in more conventional wavelength bands such as optical and X-ray astronomy. The discovery of VHE gamma-rays from such sources suggests that they may be `dark accelerators’, as Stefan Funk from the Max-Planck Institut in Heidelberg affirms: “These objects seem to only emit radiation in the highest energy bands. We had hoped that with a new instrument like H.E.S.S. we would detect some new sources, but the success we have now exceeds all our expectations.”

Dr Paula Chadwick of the University of Durham adds “Many of the new objects seem to be known categories of sources, such as supernova remnants and pulsar wind nebulae. Data on these objects will help us to understand particle acceleration in our galaxy in more detail; but finding these ‘dark accelerators’ was a surprise. With no counterpart at other wavelengths, they are, for the moment, a complete mystery.”

Cosmic particle accelerators are believed to accelerate charged particles, such as electrons and ions, by acting on these particles with strong shock waves. High-energy gamma rays are secondary products of the cosmic accelerators and are easier to detect because they travel in straight lines from the source, unlike charged particles which are deflected by magnetic fields. The cosmic accelerators are usually visible at other wavelengths as well as VHE gamma rays.

The H.E.S.S. array is ideal for finding these new VHE gamma ray objects, because as well as studying objects seen at other wavelengths that are expected to be sources of very high energy gamma rays, its wide field of view (ten times the diameter of the Moon) means that it can survey the sky and discover previously unknown sources.

Another important discovery is that the new sources appear with a typical size of the order of a tenth of a degree; the H.E.S.S. instrument for the first time provides sufficient resolution and sensitivity to see such structures. Since the objects cluster within a fraction of a degree from the plane of our Galaxy, they are most likely located at a significant distance – several 1000 light years from the sun – which implies that these cosmic particle accelerators extend over a size of light years.

The results were obtained using the High Energy Stereoscopic System (H.E.S.S.) telescopes in Namibia, in South-West Africa. This system of four 13 m diameter telescopes is currently the most sensitive detector of VHE gamma-rays, radiation a million million times more energetic than the visible light. These high energy gamma rays are quite rare – even for relatively strong sources, only about one gamma ray per month hits a square meter at the top of the earth’s atmosphere. Also, since they are absorbed in the atmosphere, a direct detection of a significant number of the rare gamma rays would require a satellite of huge size. The H.E.S.S. telescopes employ a trick – they use the atmosphere as detector medium. When gamma rays are absorbed in the air, they emit short flashes of blue light, named Cherenkov light, lasting a few billionths of a second. This light is collected by the H.E.S.S. telescopes with big mirrors and extremely sensitive cameras and can be used to create images of astronomical objects as they appear in gamma-rays.

The H.E.S.S. telescopes represent several years of construction effort by an international team of more than 100 scientists and engineers from Germany, France, the UK, Ireland, the Czech Republic, Armenia, South Africa and the host country Namibia. The instrument was inaugurated in September 2004 by the Namibian Prime Minister, Theo-Ben Guirab, and its first data have already resulted in a number of important discoveries, including the first astronomical image of a supernova shock wave at the highest gamma-ray energies.

Original Source: PPARC News Release

Image credit: Hubble
University scientists have co-founded an international collaboration that seeks to measure with new precision the mysterious force causing the universe to fly apart. Plans call for the project, named the Dark Energy Survey, to collect data on approximately 300 million galaxies spanning two-thirds of the history of the universe.

The survey could begin making observations as early as the fall of 2009. Although the DES remains more than four years away, more ambitious surveys will take at least a decade to produce results. ?I don?t want to wait that long,? said Joshua Frieman, Professor in Astronomy & Astrophysics and the College.

According to physics accounting methods, dark energy makes up 70 percent of the universe. Dark energy might be a manifestation of Albert Einstein?s cosmological constant, a force that acts at all times and in all places throughout the universe. It might also be a breakdown of Einstein?s theory of gravity on vast scales.

?It essentially requires gravity to be repulsive,? said Wayne Hu, Associate Professor in Astronomy & Astrophysics. ?That?s possible under our standard theories of gravity, but it?s not expected.? Whatever dark energy is, Frieman said, ?it?s likely to have profound implications for fundamental physics.?

The DES collaboration consists of researchers at Chicago, Fermi National Accelerator Laboratory, the University of Illinois at Urbana-Champaign, the Lawrence Berkeley National Laboratory and the Cerro Tololo Inter-American Observatory, as well as groups from the United Kingdom and Barcelona, Spain. Funding for the $20 million project is likely to come primarily from the U.S. Department of Energy, European funding agencies, the member institutions, and other agencies and sources.

Frieman heads the University?s component of the collaboration. Joining him and Hu in the collaboration are John Carlstrom, the S. Chandrasekhar Distinguished Service Professor in Astronomy & Astrophysics and the College; Scott Dodelson, Professor in Astronomy & Astrophysics and the Physical Sciences Collegiate Division; Stephen Kent, Associate Professor in Astronomy & Astrophysics; Erin Sheldon, Fellow in the Kavli Institute for Cosmological Physics; and Risa Wechsler, Hubble Fellow in the Kavli Institute for Cosmological Physics. Frieman and Dodelson also are members of Fermilab?s Theoretical Astrophysics Group, which Dodelson heads, while Kent heads Fermilab?s Experimental Astrophysics Group.

The DES will entail installing a 520-megapixel camera on the existing four-meter Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile. ?This would be larger than any existing optical camera in the world,? Frieman said.

A few hundred megapixels may not sound like much, Frieman said, ?but they?re not the same pixels that go into your hand-held. They have much higher sensitivity. They?re high-precision, high-efficiency detectors.? Furthermore, the camera will allow the scientists to survey the sky 10 times faster than they could at any existing U.S. observatory.

?The camera that?s now on the telescope just has too small a field of view. It would take us many decades to do the survey,? Frieman said.

The new camera will enable the DES to employ four techniques in attempting to discriminate between the two broad explanations for dark energy?the cosmological constant or a breakdown of gravity.

?The first method and the one that really drives the survey design is to count clusters of galaxies,? Frieman said. In this effort it will work in tandem with Carlstrom?s South Pole Telescope, which is scheduled to begin making observations in March 2007.

The SPT will help reveal if dark energy has suppressed the formation of galaxy clusters over the history of the universe. A radio telescope, the SPT will detect galaxy clusters by the way they distort the microwave radiation left over from the big bang. If theorists know how distant and how massive the galaxy clusters are, they can predict how many there should be in the presence of dark energy. The DES will make optical measurements to estimate their distance through the colors of the galaxies and their mass by gravitational lensing, the distortion of light by an intervening galaxy cluster. ?That?s a really elegant test,? Hu said.

The third technique employs gravitational lensing on a cosmic scale. Theorists can predict the effect of the dark energy on the large-scale distribution of the dark matter. With its large survey area, the DES can measure the tiny distortion of the images of galaxies induced by fluctuations in the dark matter density.

The fourth method involves the same technique that led to the 1998 discovery of dark energy: measuring the distance to a certain type of exploding star to reconstruct the expansion history of the universe. Astronomers studied these exploding stars expecting to find that the expansion of the universe had slowed as time went on. They discovered instead an accelerated expansion.

?These techniques complement each other very well,? Frieman said. ?They suffer from different sources of error, so if they agree, that gives you confidence in your result.?

For his part, Hu hopes the tests will reveal some discrepancy between predictions and reality. ?To me that would be the most exciting thing.?

Original Source: University of Chicago News Release

Super star clusters are groups of hundreds of thousands of very young stars packed into an unbelievably small volume. They represent the most extreme environments in which stars and planets can form.

Until now, super star clusters were only known to exist very far away, mostly in pairs or groups of interacting galaxies. Now, however, a team of European astronomers [1] have used ESO’s telescopes to uncover such a monster object within our own Galaxy, the Milky Way, almost, but not quite, in our own backyard!

The newly found massive structure is hidden behind a large cloud of dust and gas and this is why it took so long to unveil its true nature. It is known as “Westerlund 1” and is a thousand times closer than any other super star cluster known so far. It is close enough that astronomers may now probe its structure in some detail.

Westerlund 1 contains hundreds of very massive stars, some shining with a brilliance of almost one million suns and some two-thousand times larger than the Sun (as large as the orbit of Saturn)! Indeed, if the Sun were located at the heart of this remarkable cluster, our sky would be full of hundreds of stars as bright as the full Moon. Westerlund 1 is a most unique natural laboratory for the study of extreme stellar physics, helping astronomers to find out how the most massive stars in our Galaxy live and die.

From their observations, the astronomers conclude that this extreme cluster most probably contains no less than 100,000 times the mass of the Sun, and all of its stars are located within a region less than 6 light-years across. Westerlund 1 thus appears to be the most massive compact young cluster yet identified in the Milky Way Galaxy.

Super Star Clusters
Stars are generally born in small groups, mostly in so-called “open clusters” that typically contain a few hundred stars. From a wide range of observations, astronomers infer that the Sun itself was born in one such cluster, some 4,500 million years ago.

In some active (“starburst”) galaxies, scientists have observed violent episodes of star formation (see, for example, ESO Press Photo 31/04), leading to the development of super star clusters, each containing several million stars.

Such events were obviously common during the Milky Way’s childhood, more than 12,000 million years ago: the many galactic globular clusters – which are nearly as old as our Galaxy (e.g. ESO PR 20/04) – are indeed thought to be the remnants of early super star clusters.

All super star clusters so far observed in starburst galaxies are very distant. It is not possible to distinguish their individual stars, even with the most advanced technology. This dramatically complicates their study and astronomers have therefore long been eager to find such clusters in our neighbourhood in order to probe their structure in much more detail.

Now, a team of European astronomers [1] has finally succeeded in doing so, using several of ESO’s telescopes at the La Silla observatory (Chile).

Westerlund 1
The open cluster Westerlund 1 is located in the Southern constellation Ara (the Altar constellation). It was discovered in 1961 from Australia by Swedish astronomer Bengt Westerlund, who later moved from there to become ESO Director in Chile (1970 – 74). This cluster is behind a huge interstellar cloud of gas and dust, which blocks most of its visible light. The dimming factor is more than 100,000 – and this is why it has taken so long to uncover the true nature of this particular cluster.

In 2001, the team of astronomers identified more than a dozen extremely hot and peculiar massive stars in the cluster, so-called “Wolf-Rayet” stars. They have since studied Westerlund 1 extensively with various ESO telescopes.

They used images from the Wide Field Imager (WFI) attached to the 2.2-m ESO/MPG as well as from the SUperb Seeing Imager 2 (SuSI2) camera on the ESO 3.5-m New Technology Telescope (NTT). From these observations, they were able to identify about 200 cluster member stars.

To establish the true nature of these stars, the astronomers then performed spectroscopic observations of about one quarter of them. For this, they used the Boller & Chivens spectrograph on the ESO 1.52-m telescope and the ESO Multi-Mode Instrument (EMMI) on the NTT.

An Exotic Zoo
These observations have revealed a large population of very bright and massive, quite extreme stars. Some would fill the solar system space within the orbit of Saturn (about 2,000 times larger than the Sun!), others are as bright as a million Suns.

Westerlund 1 is obviously a fantastic stellar zoo, with a most exotic population and a true astronomical bonanza. All stars identified are evolved and very massive, spanning the full range of stellar oddities from Wolf-Rayet stars, OB supergiants, Yellow Hypergiants (nearly as bright as a million Suns) and Luminous Blue Variables (similar to the exceptional Eta Carinae object – see ESO PR 31/03).

All stars so far analysed in Westerlund 1 weigh at least 30-40 times more than the Sun. Because such stars have a rather short life – astronomically speaking – Westerlund 1 must be very young. The astronomers determine an age somewhere between 3.5 and 5 million years. So, Westerlund 1 is clearly a “newborn” cluster in our Galaxy!

The Most Massive Cluster
Westerlund 1 is incredibly rich in monster stars – just as one example, it contains as many Yellow Hypergiants as were hitherto known in the entire Milky Way!

“If the Sun were located at the heart of Westerlund 1, the sky would be full of stars, many of them brighter than the full Moon”, comments Ignacio Negueruela of the Universidad de Alicante in Spain and member of the team.

The large quantity of very massive stars implies that Westerlund 1 must contain a huge number of stars. “In our Galaxy, explains Simon Clark of the University College London (UK) and one of the authors of this study, “there are more than 100 solar-like stars for every star weighing 10 times as much as the Sun. The fact that we see hundreds of massive stars in Westerlund 1 means that it probably contains close to half a million stars, but most of these are not bright enough to peer through the obscuring cloud of gas and dust”. This is ten times more than any other known young clusterin the Milky Way.

Westerlund 1 is presumably much more massive than the dense clusters of heavy stars present in the central region of our Galaxy, like the Arches and Quintuplet clusters. Further deep infrared observations will be required to confirm this.

This super star cluster now provides astronomers with a unique perspective towards one of the most extreme environments in the Universe. Westerlund 1 will certainly provide new opportunities in the long-standing quest for more and finer details about how stars, and especially massive ones, do form.

… and the Most Dense
The large number of stars in Westerlund 1 was not the only surprise awaiting Clark and his colleagues. From their observations, the team members also found that all these stars are packed into an amazingly small volume of space, indeed less than 6 light-years across. In fact, this is more or less comparable to the 4 light-year distance to the star nearest to the Sun, Proxima Centauri!

It is incredible: the concentration in Westerlund 1 is so high that the mean separation between stars is quite similar to the extent of the Solar System.

“With so many stars in such a small volume, some of them may collide”, envisages Simon Clark. “This could lead to the formation of an intermediate-mass black hole more massive than 100 solar masses. It may well be that such a monster has already formed at the core of Westerlund 1.”

The huge population of massive stars in Westerlund 1 suggests that it will have a very significant impact on its surroundings. The cluster contains so many massive stars that in a time span of less than 40 million years, it will be the site of more than 1,500 supernovae. A gigantic firework that may drive a fountain of galactic material!

Because Westerlund 1 is at a distance of only about 10,000 light-years, high-resolution cameras such as NAOS/CONICA on ESO’s Very Large Telescope can resolve its individual stars. Such observations are now starting to reveal smaller stars in Westerlund 1, including some that are less massive than the Sun. Astronomers will thus soon be able to study this exotic galactic zoo in great depth.

More information
The research presented in this ESO Press Release will soon appear in the leading research journal Astronomy and Astrophysics (“On the massive stellar population of the Super Star Cluster Westerlund 1” by J.S. Clark and colleagues). The PDF file is available at the A&A web site. A second paper (“Further Wolf-Rayet stars in the starburst cluster Westerlund 1”, by Ignacio Negueruela and Simon Clark) will also soon be published in Astronomy and Astrophysics. It is available as astro-ph/0503303.
A Spanish press release issued by Universidad de Alicante is available on the web site of Ignacio Negueruela.

Note
[1]: The team is composed of Simon Clark (University College London, UK), Ignacio Negueruela (Universidad de Alicante, Spain), Paul Crowther (University of Sheffield, UK), Simon Goodwin (University of Wales, Cardiff, UK), Rens Waters (University of Amsterdam) and Sean Dougherty (Dominion Radio Astrophysical Observatory).

Original Source: ESO News Release